The present invention concerns a continuous process for oxidation. This process is particularly suitable for the oxidation step in the anthraquinone process for production of hydrogen peroxide, in which substituted anthrahydroquinones are oxidized by oxidation with an oxygen-containing gas to form substituted anthraquinones and hydrogen peroxide.
In the anthraquinone process this oxidation was initially performed cocurrently in gas distribution towers connected in series using fresh air in each tower, which was both technically complicated and uneconomic. Although the oxidation rate could be increased in a cocurrent flow of air and working solution according to U.S. Pat. No. 3,073,680 whilst retaining specific bubble sizes, obtainable by means of fine-pore gas distributor organs and specific cross-sectional loads, problems arose with the removal of the resulting foam and with the gas-liquid phase separation.
U.S. Pat. No. 2,902,347 describes the performance of oxidation using a countercurrent process. The hydroquinone solution to be oxidized is charged into the top of a packed column and flows countercurrently to the rising air metered into the base of the column. The disadvantage of this type of countercurrent flow is a very low flooding limit in the column, which means that several columns have to be connected in series in order to achieve as complete a conversion as possible.
DE-A-20 03 268 describes a combination of cocurrent and countercurrent flow. The problems cited above can be resolved by means of an oxidation column divided into two to six sections. In each section of this column the working solution and oxidation gas flow cocurrently from bottom to top, but viewed from the column as a whole the gas and liquid move countercurrently to each other. The disadvantage of this cascade-type arrangement is the pressure drop due to fittings such as perforated plates, gauze or packing bodies, which are necessary for intimate and thorough mixing.
In order to reduce the pressure drop in the cascade-type column arrangement described above, EP-A-221 931 suggests performing the oxidation in a cocurrent reactor with no fittings. The hydrogenated working solution and oxidation gas produce a coalescence-inhibited system in which the gas bubbles, once formed, retain their size in the absence of any external influence. The disadvantage of this design proved to be that the reactor volume to be aerated (m3 per t H2O2) is quite large, which leads to a lower space-time yield and to a high hold-up of expensive working solution. In addition the cocurrent flow leads to an increased proportion of degradation products.
An object of the present invention is therefore to increase the efficiency of a continuous oxidation consisting of the reaction mixture comprising the substance to be oxidized and the oxidizing gas.
The above and other objects of the present invention can be achieved by passing the liquid medium containing the substance to be oxidized and the oxidizing gas countercurrently during the continuous oxidation, whereby the oxidizing gas is mixed with a split stream containing preoxidized or partially oxidized substance before it enters the reactor, which is designed to be substantially free from coalescence-promoting fittings other than at least one distributor organ.
This result is surprising, since recycling a preoxidized split stream of product would normally reduce the space-time yield in the reactor. In fact the effect of mixing this split product stream with the oxidation gas before it enters the reactor, which is designed to be substantially free from coalescence-promoting fittings, is that the bubbles generated in the mixing organ retain their set size. This results in a reaction rate across the entire volume of the unencumbered reactor that is significantly higher than that achieved without premixing. This more than compensates for the loss in space-time yield arising from the recycling of a split product stream.
At the same time the countercurrent flow suppresses the formation of byproducts such as is known from a cocurrent flow. The premixing also increases the degree of oxidation. By bringing the split product stream into contact with fresh oxidation gas, further portions of unoxidized substance are reacted.
In order to maintain the bubble size set in the mixing organ during the course of the reaction, oxidation reactors are used that are free from coalescence-promoting fittings. Plate-type heat exchangers can optionally be fitted, particularly in order to achieve an isothermic reaction process. The stacks of heat exchangers should be fitted in such a way that they do not interrupt the medium in its direction of flow in the reactor, and the gap width of a plate-type heat exchanger should be chosen such that it does not cause any coalescence.